The present invention relates to a wind power generation apparatus, and in particular, to an apparatus and a method for protecting a converter of the wind power generation apparatus from overcurrent, etc. when a voltage drop (power outage, etc) occurs to an electric power system which is connected to the wind power generation apparatus.
In an AC-excited generator employed for a power generation apparatus, the frequency of voltage caused by the rotor excitation to the stator-side of the generator can be made equal to system frequency, by making the power converter excite the rotor winding with AC current at a slip frequency (the difference between the system frequency and the frequency of revolution). Making the rotor excitation frequency (or the slip frequency) variable has various advantages. For example, the revolving speed of the wind turbine of the wind power generation apparatus can be made variable, while also making the capacity of the power converter smaller than that of the generator.
However, when a voltage drop (ground fault, etc.) occurs in the power system connected to the wind power generation apparatus, the AC-excited generator operates so as to supply electric current to the point of the failure. In this case, excessive current is induced in the rotor winding, causing excessive current passing through the excitation converter connected to the rotor-side of the generator. In order to prevent trouble caused by the excessive current (overcurrent), a device called “crowbar”, for short-circuiting the rotor circuit by use of a thyristor, is used, for example.
In recent years, standards, specifying that the operation of a wind power generation apparatus should be continued even in the event of the power system failure without disconnecting the wind power generation system from the power system, have been set up in Europe, etc. Under such circumstances, system operation capable of minimizing ill effects on the power system is being desired.
The AC-excited generators have been used mainly for large-scale power generation systems (e.g. pumped-storage power plants), for which an operation method, short-circuiting the rotor circuit by use of externally commuted elements in the event of major power outage in the system, has been employed so far.
An example of a conventional power generation system is disclosed in “400-MW Adjustable-Speed Pumped-Storage Hydraulic Power Plant” by Eizo Kita, et al., HITACHI REVIEW, 1995, Vol. 44. JP-A-2006-230085 and JP-A-2007-244136 disclose a power conversion device for exciting a secondary excitation type generator in which a power converter is protected from overcurrent to secure the operation of the power converter.